Nanotechnology is poised to be one of the primary technologies of the future. Nanoparticles, particles having a size of between 1 and 100 nm, find applications in various fields of research and industry. Nanoparticles are used as biomarkers, as catalysts, for drug delivery, as antibacterial materials, and in printable electronics, such as conductive inks.
In particular, metal nanoparticles are becoming important products in the chemical industry. The main reason for interest in nanometals between 1 to 50 nm in size is their high surface areas. It is estimated that 10% of metal atoms are on the surface for a particle with a 10 nm diameter. In comparison, 60% of the atoms are on the surface for a 2.5 nm size particle, R Bönnemann, R. M Richards, Nanoscopic metal particles-synthetic methods and potential applications, Eur. J. Inorg. Chem., 2455-2480 (2001). Having such a high percentage of atoms exposed at the surface gives nanometals a distinct advantage over bulk materials. Nanometals are used as catalysts, antibacterial, agents, electrical conductors in printed circuits, sensors, magnetic materials, components of composites, components of fuel cells and biochemical analysis, and in many other areas and applications. Silver, copper, nickel, cobalt, gold, titanium, platinum, and iron nanoparticles are already in use. According to the Woodrow Wilson International Center for Scholars' “The Nanotechnology Consumer Products Inventory,” published in March of 2006, there are already 212 nanoproducts on the consumer market. A large part of the market is occupied by nanosilver, which consists of 25 products. Only carbon nanotubes and fullerens have more products, 29, on the market. Nanosilver is used in anti-wrinkle and antibacterial clothes (soldier underpants, socks), antibacterial wound dressing, water treatment, and electrical contacts.
Another advantage of nanoparticles is that they provide the same or better quality of product using less material. For example, using nanoparticles in the production of printable electronics results in less material being used. Lines for printed circuits are thinner (0.1-0.5 micron compared to approximately 10 microns in traditional techniques) and narrower (10-50 microns). Accordingly, this significantly reduces the material used on the printed circuit by approximately 10 times, resulting in lowered production cost. As the technology opens up new market opportunities with greater demand and material purchases, the electronics industry will continue seeking low cost raw materials to support new applications.
Potentially, metal nanoparticles can replace large size particles in many applications. Thus, consumers will receive the same or better quality product, using smaller amounts of materials, resulting in lower costs.
Currently, nanometals are produced in small quantities by reduction of metal salts in water or organic solutions. Particles obtained using these techniques normally have consistent size distributions, but the size is typically large (greater than 20 nm) and not well controlled.
A variety of techniques have been proposed, and/or reduced to practice, for the synthesis of nanoparticles, including: arrested precipitation in solutions, synthesis in structured medium, high temperature pyrolysis, sonochemical and radiochemical methods and others. See A. P. Alivisatos, Perspectives on the Physical Chemistry of Semiconductor Nanocrystals, J. Phys. Chem. 100(31), 13226-13239 (1996); A. Eychmuller, Structure and Photophysics of Semiconductor Nanocrystals, J. Phys. Chem. 104(28), 6514-6528 (2000); C. B. Murray, C. R. Kagan, M. G. Bawendi, Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies, Ann. Rev. Mater. Sci. (30), 545-610 (2000); M. Green, P. O'Brien, Recent Advances in the Preparation of Semiconductors as Isolated Nanometric Particles: New routes to Quantum Dots, Chem. Commun., 2235-2241 (1999); T. Trindadae, P. O'Brien, N. L. Pickett, Nanocrystalline Semiconductors: Synthesis, Properties and Perspectives, Chem. Mater. 13, 3843-3858 (2001). Current industrial technologies for the production of nano-oxides and nanometals in large quantities involve high temperature preparation of particles in a gas phase, without surfactants to stabilize particles during the growth. Rather, nanometals are produced in the gas phase by vacuum evaporation techniques and then stabilized in solution using standard surfactants. As a result, nanoparticles created using these methods tend to agglomerate, their shelf life is limited, and the shapes and the sizes of nanoparticles are not well controlled. These particles can be stabilized after suspending them in solution using surfactants and ultrasonic irradiation for dispersion of aggregates. This procedure does not greatly change the size and shape, but improves the stability of particles in solution.
Solution synthesis of metal nanoparticles dates back to 1857 when Faraday published a paper on the synthesis of zero-valent metals by reduction in the presence of surface stabilizing agents. This method has become prevalent since that time, and modifications are aimed at the improvements to the size control and size distribution of nanoparticles. Metal colloids are considered to be “monodisperse” if the size distribution deviates less than 15% from the average size value. “Narrow size distribution” usually means that the particle size histogram has a standard deviation, σ, smaller than 20%. Typically, the size distribution is not very good for particles produced using a solution method at low temperatures. Therefore, size selection methods are necessary to achieve the desired nanoparticle quality, see H. Bönnemann, R. M Richards, Nanoscopic metal particles-synthetic methods and potential applications, Eur. J. Inorg. Chem., 2455-2480 (2001). The “citrate” method, developed by Wilcoxon and Brust, Wilcoxon, J. P., Williamson, R. L., Baughman, R., Optical Properties of Gold Colloids Formed in Inverse Micelles, J. Chem. Phys. 98, 9933-9950; Brust, M., Walker, M., Bethel, D., Schiffrin, D. J., Whyman, R., J. Chem. Soc., Chem Commun., 801-802 (1994), produces good quality nanoparticles, but it is restricted to using water as a solvent and it is unable to produce a high concentration of nanometal in solution. Currently, the best technique for producing high-quality semiconductor quantum dots, nano-oxides, and nanometals is a high temperature pyrolysis of precursors in high boiling point solvents. See A. P. Alivisatos, Perspectives on the Physical Chemistry of Semiconductor Nanocrystals, J. Phys. Chem. 100(31), 13226-13239 (1996); A. Eychmuller, Structure and Photophysics of Semiconductor Nanocrystals, J. Phys. Chem. 104(28), 6514-6528 (2000); C. B. Murray, C. R. Kagan, M. G. Bawendi, Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies, Ann. Rev. Mater. Sci. (30), 545-610 (2000); M. Green, P. O'Brien, Recent Advances in the Preparation of Semiconductors as Isolated Nanometric Particles: New routes to Quantum Dots, Chem. Commun., 2235-2241 (1999); T. Trindadae, P. O'Brien, N. L. Pickett, Nanocrystalline Semiconductors: Synthesis, Properties and Perspectives, Chem. Mater. 13, 3843-3858 (2001).
There are many variants of this organometallic route, and great progress has been achieved in the synthesis of semiconductor quantum dots. The synthesis of cadmium chalcogenides is the best developed and produces highly fluorescent nanoparticles with a narrow size distribution. Work on the synthesis of nanometals is in progress and some synthetic procedures are described below.
Pyrolysis of metal carbonyls has been used for the production of metal nanoparticles like cobalt, iron, nickel, and others, but with a relatively large size distribution, see V. F. Puntes, K. M. Krishnan, A. P. Alivasatos, Colloidal Nanocrystal Shape and Size Control: The Case of Cobalt, Science 291, 2115-2117 (2001). Careful control of the ligands' nature and the combination of surfactants improve control of the size distribution, C. S. Samia, K. Hyzer, J. A Schlueter, C. J. Qin, J. S. Jiang, S. D. Bader, X. M. Lin, Ligand Effect on the Growth and the Digestion of Co Nanocrystals, J. Am. Chem. Soc. 127, 4126-4127 (2005). Other approaches use a weaker reducing agent and a more stable precursor. Using this method, see S. SD. Bunge, T. J. Boyle, T. J. Headley, Synthesis of Coinage-Metal Nanoparticles from Mesityl Precursors, Nano Lett. 3, 901-905 (2003), copper, silver, and gold mesityl complexes were dissolved in octylamine and then subsequently injected in hot (300° C.) hexadecylamine. This method uses expensive precursors and is not suitable for large scale production.
In another paper, see N. R. Jana, X. Peng, Single-Phase and Gram-Scale Routes Toward Nearly Monodisperse Au and Other Noble Metal Nanocrystals, J. Am. Chem. Soc. 125, 14280-14281 (2003), gold chloride, silver acetate, copper acetate, or platinum chloride was dissolved in toluene with ammonium surfactant. Either tetrabutylammonium borohydride or its mixture with hydrazine in toluene was used as the reducing agent. Fatty acids or aliphatic amines served as ligands. The drawback of their approach is that authors of paper used expensive chemicals as solvents, didodecyldimethylammonium bromide, lengthy and complicated procedures, including sonication to allow dissolution of precursors.
Hiramatsu and Osterloh, see H. Hiramatsu, E. Osterloh, A Simple Large-Scale Synthesis of Nearly Monodisperse Gold and Silver Nanoparticles with Adjustable Sizes and with Exchangeable Surfactants, Chem. Mater. 16, 2509-2511 (2004), described an inexpensive and reproducible method for the synthesis of organoamine stabilized gold and silver nanoparticles in the 6-21 nm (gold) and 8-32 nm (silver) size ranges with polydispersities as low as 6.9%. Their procedure requires only three reagents: tetrachloroauric acid or silver acetate, oleylamine, and a solvent. The reaction proceeds in 2 hours in toluene under refluxing and produces good quality gold or silver nanoparticles. This procedure however requires lengthy time and heating.
NanoMas Technologies, owner of the rights to Pub. No.: WO/2007/120756, 25 Oct. 2007, disclosing the method of synthesis of silver nanoparticles in a two-phase system, where silver acetate is dissolved in organic solvent (toluene) and sodium borohydride is dissolved in water. Then sodium borohyrdide is added to silver precursor and the reaction mixture is stirred for 2.5 hours. The water phase, is removed by separation funnel and toluene is concentrated by rotor-evaporator. Then silver nanoparticles are precipitated by addition of methanol. Their procedure includes the presence of water in the reaction mixture, which can absorb on the surface of nanosilver and deteriorate its properties. The procedure is tedious and includes undesirable steps, like water separation, etc. Silver acetate is used as a silver precursor, which is more expensive than silver nitrate.
Xerox, owner of the rights to US Patent Application 20060073667 disclosing a method of synthesis which includes dissolving, at 60 C, silver acetate in toluene in the presence of dodecylamine as surfactant and then adding phenylhydrazine to the mixture. The solution is kept at 60 C for 1 hour and then cooled down. This procedure is lengthy, includes expensive silver precursor, and heating. Dodecylamine is not a good surfactant as it does not provide long-term stability for the silver nanoparticles.